The present invention relates to a pigment grade zinc cyanamide and manufacturing procedure, characterized by high assay, accordingly high corrosion inhibitive activity and more particularly, however, by crystal structure and properties substantially different from those of the prior art.
Zinc cyanamide is known in the prior art for its valuable corrosion inhibiting properties and its applicability as a pigment component of primer formulations and, specifically, in mirror backing protective coatings employed on metallic silver substrates. Pigment grade qualities are preferably produced by wet procedures which yield products characterized usually by high assay, uniform and narrow particle size distribution, high specific surface area and, consequently, by enhanced corrosion inhibitive activity in protective coatings.
Basically, there are two versions of wet manufacturing procedures known by the prior art: By precipitation, ZnNCN can be produced according as follows: EQU ZnX+Na.sub.2 NCN.fwdarw.ZnNCN+Na.sub.2 X 1.
Where ZnX is soluble, preferably ZnSO.sub.4 and eventually Na.sub.2 NCN is prepared "in situ" by H.sub.2 NCN and NaOH addition.
If performed in ammoniacal medium, due to the presence of ammonium salts formed as byproducts, the ZnNCN precipitation occurs incompletely and as a consequence, the resulting process waters are environmentally incompatible, according to the following equilibrium: EQU [Zn(NH.sub.3).sub.4 ].sup.2+ x.sup.2- +H.sub.2 NCN+2H.sub.2 O.revreaction.ZnNCN+(NH.sub.4).sub.2 x+2NH.sub.4 OH 2.
It is known that generally the presence of soluble salt contaminants severely affects the quality of inhibitor pigment grade products. Consequently, the formation of soluble byproducts (See Equations 1 and 2) constitutes a considerable shortcoming of manufacturing procedures based on precipitation reactions.
The above mentioned shortcomings are avoided by procedures based on heterogeneous processes, as for example, the direct conversion of hydrated zinc oxide into zinc cyanamide, pursuant to the following: EQU ZnO+H.sub.2 O.fwdarw.Zn(OH).sub.2 3. EQU Zn(OH).sub.2 +H.sub.2 NCN.fwdarw.ZnNCN+H.sub.2 O 4.
Reaction 4 takes place on the solid-liquid interface, presumably by a nucleophilic attack of the .sup.2(-) NCN anion which does form under the mildly alkaline conditions of the Zn(OH).sub.2 slurry.
Pigment grade ZnNCN applicable in protective coatings (i.e., mirror backing) is obtainable with high assay and virtually soluble salt-free conditions according to such direct procedure disclosed by U.S. Pat. No. 5,176,894.
Corrosion, even of coated metal surfaces, is a thermodynamically favored electrochemical process supported by moisture, always present in organic coatings at equilibrium with the humidity of the surroundings. In such conditions, diffusion controlled transport processes of available soluble species occur continuously between all involved interfaces of the substrate/coating system. Such soluble species transported by diffusion processes are generally airborne corrosion promoters or may be generated "in situ" by soluble components of the coating system.
In reference to ZnNCN, it is the .dbd.N--C.dbd.N moiety of the pigment composition which, by continuous solubilization, generates the electrochemically active species characterized by substrate-specific inhibitor activity displayed, as for example, on silver. By contrast, if soluble inorganic salt contaminants accompany the host pigment, corrosion promoter ionic species are generated "in situ" which tend to obliterate the protective effect of the inhibitor. Consequently, it becomes evident that high assay and soluble salt-free conditions are necessary quality requirements for pigment grade zinc cyanamides intended for protective coating formulations. However, in addition to chemical quality parameters, several physical parameters need to be considered in characterizing such valuable end-use properties of pigment grade products as rheological behavior in paint systems and, specifically, corrosion inhibitive activity.
Considering the dynamic nature of the corrosion process, it is important to specify that the necessary condition of effective corrosion inhibition is to maintain a critical or higher dynamic equilibrium concentration of inhibitor species at the coating-substrate interface all during the useful service life of the protective coating system. In this sense, corrosion inhibitive pigments as functional components of protective coating systems, can be regarded as solid reservoirs which, by moisture supported solubilization, at an adequate rate, continuously generate "in situ" electrochemically active inhibitor species.
It is evident that corrosion inhibitive activity should be considered as a water solubility or solubility rate dependent, discriminant property of electrochemically active pigment grade products. Virtually insoluble or very soluble compounds, although of potentially adequate chemical composition, cannot function as such, however, for different reasons: lack of any inhibitive activity of the former, and due to increase of the coating's water sensitivity by the latter.
Contrary to related basic physical constants, several functional properties (i.e.: rheological characteristics, corrosion inhibitive activity) of pigment grade bulk solids are highly dependent on their variable physical conditions, determined by such microscopic qualities as particle size distribution, surface modification circumstances and, specifically, by crystal morphological characteristics. Controlled alterations of bulk solids' physical conditions which usually result in measurable value variations of such related macroscopic parameters as oil absorption, apparent bulk density and notable solubilization rate, are common practices of advanced pigment manufacturing technologies.
In this respect, alteration of crystal morphological characteristics, such as augmenting crystallite acicularity (which results in increased specific surface area and oil absorption, respectively in decreased apparent bulk density values), offers attractive possibilities to condition solubility rate, as well as the related inhibitive activity of corrosion retardant pigment grade products.